Routing

Definition

Deciding where a packet goes next based on its destination. Happens at L3 (IP) classically, but also at L4 (load balancers) and L7 (service mesh, ingress).

Where it appears

🌐 Networking

• Static — manual next-hop entries
• Dynamic — OSPF, BGP, EIGRP, IS-IS
• Policy-based routing — route by source, marking, or app

🐧 Linux

• ip route — kernel routing table
• FRR / BIRD — software routing daemons (OSPF, BGP on Linux)
• Multi-table routing — ip rule + multiple ip route tables

☁️ Cloud

• AWS VPC — route tables per subnet; IGW, NAT, TGW, VPC endpoint as targets
• Azure VNet — system routes + UDRs (user-defined routes); NVA pattern
• Transit Gateway / Virtual WAN — hub-and-spoke at scale

📦 Containers

• Kubernetes CNI — Calico, Cilium, Flannel each handle pod routing differently
• Service mesh — L7 routing via Envoy sidecars

See also

• Segmentation
• DNS
• L3